The Avogadro experiment and the watt balance are the pillars of the redefinition of the kilogram. They represent two independent possibilities of realizing the redefinition. Today, the unit "kilogram" is defined as the mass of the international prototype of the kilogram which is kept in a safe in Sèvres near Paris. International efforts are aimed at relating all base units, such as the second, the meter, the kilogram and the ampere, in future only to fundamental constants in order to create a system of units which is independent of artifacts. For this purpose, it is determined in the Avogadro experiment how many atoms are contained in almost perfect silicon spheres. The next possibility for a redefinition of the kilogram will present itself on the occasion of the General Conference on Weights and Measures (Conférence Générale des Poids et Mesures, CGPM) in 2018. At the same time, the results of the Avogadro experiment must beconsistent with those of the watt balance. It is only then that a safe and exact redefinition of the unit of mass is guaranteed. The realization of the Avogadro experiment is based on the contributions of different working groups of PTB and of national and international partners.
Trailer zum Avogadro-ProjektTrailer zum Avogadro-Projekt
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Ein Wissenschaftler-Team der Physikalisch-Technische Bundesanstalt (PTB) erhält am 8. November 2019 den Technologietransferpreis der Industrie- und Handelskammer Braunschweig für die gelungene Weitergabe einer mess- und fertigungstechnischen Meisterleistung aus der Forschung in die Industrie. Die Rede ist von hochreinen, extrem runden Siliziumkugeln, die die neue Definition des Kilogramms...
This revolution has been a long time in the making. Behind it are metrologists working in their scientific laboratories, and scientific managers in their decision-making committees. Now the moment is approaching when many of the old quantities (specifically, the definitions behind them) will have to step down to make way for their successors, who are waiting in the wings. The quantities concerned...
In order to become independent of the international prototype of the kilogram kept in Paris, the definition of the SI unit of mass, the kilogram, has to be revised. For this purpose, the number of silicon atoms in a monocrystalline silicon sphere has been determined at PTB within the scope of the Avogadro proj´ect. It is therefore necessary to measure the surface properties with the highest...
Nach fast 130 Jahren wird das internationale Ur-Kilogramm im Herbst 2018 voraussichtlich in den wohlverdienten Ruhestand geschickt. Dann kommt die Welt zu einem metrologischen Gipfeltreffen zusammen, um über die Neudefinition des Kilogramms zu entscheiden. Wie der Meter und die Sekunde soll künftig auch das Kilogramm über unveränderliche Naturkonstanten definiert werden. Dazu zählen...
Für die akkurate Berechnung des durch Beugung verursachten Längenfehlers in der Laserinterferometrie ist eine genaue Charakterisierung der verwendeten Laserstrahlen erforderlich. Für die Messungen am Avogadroprojekt der PTB wird ein kalibrierter Wellenfrontsensor verwendet. Da dieser nur an Luft arbeitet, die Wellenfronten aber für Experimente im Vakuum bekannt sein müssen, wurde ein...
Working Group 3.44 "Avogadro Constant" coordinates the research activities at PTB and the cooperation with institutes all over the world for the precise determination of the Avogadro constant. This is to create a foundation for the redefinition of the unit of mass "kilogram" on the basis of a fundamental constant.
Preparation of the spheres
The material of the spheres was manufactured in Russia by the Central Design Bureau of Machine Building in St. Petersburg and by the Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences in Nishni Novgorod. Then, single crystals without dislocations were drawn in the Leibniz-Institut für Kristallzüchtung (IKZ) in Berlin, and from these, the spheres were manufactured. WG 4.33 "X-ray Optics" prepares the samples of the 28Si crystal, and WG 5.56 "Manufacturing Technology" manufactures the spheres with as little a form deviation as possible.
Determination of the isotopic ratio and of the molar mass
The isotopic ratio and the molar mass are measured at PTB's Working Group 3.11 "Inorganic Analysis" by means of isotope dilution mass spectrometry (IDMS). (The relative atomic masses of the naturally occurring silicon isotopes 28Si, 29Si and 30Si are known with sufficient accuracy from other measurements.)
Determination of the volume in the sphere interferometer
The volume of the silicon spheres is determined by WG 5.41
"Interferometry on Spheres" at 20 °C and in vacuum with the aid of a spherical Fizeau interferometer. In cooperation with WG 4.33 "X-ray Optics", this working group also performs numerical simulations for the interferometer. The temperature measurements of the volume and lattice parameter determinations are compared with one another and with the temperature realization of WG 7.42 "Applied Thermometry" with the aid of an electronic temperature reference point (20 °C) which has been developed by WG 3.43 "Solid State Density ".
On the surface of the spheres, an oxide layer and possibly other surface layers can be found which must be taken into account when determining the sphere volume and the sphere mass. The required knowledge of the thickness and of the structure of the surface layers on the spheres is determined by the following Working Groups with the methods indicated:
Density comparisons of the spheres and of material samples
WG 3.43 "Solid State Density" performs density comparisons of the spheres and of other samples. The density of the thermal oxide on silicon spheres has also been determined in this WG with the aid of density comparison measurements. Working Group 3.43: "Solid State Density"
Measurement of the lattice parameter
WG 4.33 "X-ray Optics" establishes X-ray interferometers for relative and absolute measurements of the lattice parameter. The optical part of the X-ray scanning interferometer is manufactured by WG 5.21 "Length and Angle Graduations".
Investigations of impurities of the sphere material
WG 3.14 "Optical Analysis" determines the contents of the main impurities boron, carbon, oxygen and nitrogen by means of low
temperature infrared spectroscopy. A result of the impurities is that the mass of the real silicon sphere differs slightly from the mass of a sphere which consists only of silicon atoms.
Working Group 3.14: Optical Analysis
Nationale und Internationale Partner
National and international partners
Dresden University: Measurement of the hydrogen concentration in the crystal by Deep Level Transient Spectroscopy (DLTS)
Halle-Wittenberg University: Measurement of the vacancy concentration in the crystal by positron annihilation experiments
Federal Institute of Metrology (METAS, Switzerland): Measurement of the surface layers of the Si spheres with X-ray photoelectron spectroscopy (XPS)
National Institute of Metrology (NIM, China), National Institute of Standards and Technology (NIST, USA) and National Research Council of Canada (NRC): Measurement of the molar mass of isotopically enriched silicon with IDMS
International Bureau of Weights and Measures (BIPM): Mass determinations of the silicon spheres in air and in vacuum and guarantee of the traceability to the international prototype of the kilogram
Istituto nazionale di ricerca metrologica (INRIM, Italy): Lattice parameter measurements at 20 °C and in vacuum with the aid of an X-ray scanning interferometer
NMIA: Diameter measurements of spheres at 20 °C and in vacuum with the aid of a Saunders interferometer
National Metrology Institute of Japan (NMIJ, Japan)
oMeasurement of the mass of the spheres in air and in vacuum
Measurement of the diameters at 20 °C and in vacuum with an interferometer
Investigation of the oxide layer thickness with spectral ellipsometry
Investigation of the homogeneity of lattice distances in the 28Si crystal
National Institute of Metrology (NIM, China): Measurements in the fields of the molar mass, the volume, the surface layers and the mass
